1. Field of the Invention
The present invention relates to noise reduction circuits, and more particularly, to a noise reduction circuit for reducing noise included in a reproduced video signal from a video signal recording/reproduction device such as a video tape recorder (referred to as a VTR hereinafter).
2. Description of the Background Art
Video signal recording/reproduction devices such as VTRs are usually provided with noise reduction circuits for reducing noise within reproduced video signals generated while video signals are recorded onto a recording medium, such as a magnetic tape, and/or reproducing video signals therefrom. Generally, a video signal comprises a luminance signal (Y signal) and a chrominance signal (C signal) interleaved with each other at the interval of 1/2.multidot.f.sub.H. In recording, such video signal is Y/C separated and the extracted luminance signal is processed by a luminance signal processing circuit to be recorded. When such luminance signal is recorded, chrominance signal components apart from a luminance signal spectrum by 1/2.multidot.f.sub.H may possibly remain in the luminance signal, and such chrominance signal components become noise components.
FIG. 1 is a block diagram schematically showing a non-recursive type noise reduction circuit which is an example of such conventional noise reduction circuits for reducing noise components.
Referring to FIG. 1, a reproduced luminance signal as a reproduced video signal, including noise, provided from a reproduction system circuit (not shown) of a VTR for example, is applied to an 1H delay line 2 implemented with a CCD delay line for example, via an input terminal 1, to be delayed by 1H period (H: horizontal period of a video signal). The delay line is not limited to 1 delay line, and may be any nH (n: natural number) delay line. The reproduced luminance signal delayed by 1H period in 1H delay line 2 is applied to a negative input terminal of a subtractor 3.
The current reproduced luminance signal provided to input terminal 1 is applied to a positive input terminal of subtractor 3, as well as to a positive input terminal of a subtractor 6. Subtractor 3 subtracts the reproduced luminance signal 1H-delayed by 1H delay line 2 from the current reproduced luminance signal which is provided from input terminal 1. Because the video information of the vertical direction and reducing noise, where the level of a signal component not having line correlation is emphasized, i.e., vertical emphasis is applied to a luminance signal to record a video signal on a magnetic tape, and the reproduced luminance signal is passed through a recursive type noise reduction circuit at the time of reproduction. A recursive type noise reduction circuit employed in such VTRs is disclosed in Japanese Patent Laying-Open No. 57-211885 and 59-158684, for example. FIG. 2 is a block diagram schematically showing such a conventional recursive type noise reduction circuit.
Referring to FIG. 2, a reproduced luminance signal, including noise, provided from a reproduction system circuit (not shown) of a VTR is applied to a positive input terminal of a subtractor 9 and a positive input terminal of a subtractor 13, via an input terminal 8. Subtractor 9 subtracts the reproduced luminance signal supplied from a 1H delay line 10 from the reproduced luminance signal to extract a component not having line correlation (mainly the noise component).
The extracted component is provided to an attenuator 12 having an arbitrary attenuation coefficient via a limiter 11, where the level is attenuated. The attenuated component is then applied to a negative input terminal of reproduced luminance signals having 1H period interval are in close approximity, i.e. have line correlation, noise components not related to video information (no line correlation) are extracted from subtractor 3.
The extracted noise component has the amplitude thereof limited via a limiter 4 to be applied to an attenuator 5 where its level is attenuated. The noise component is then applied to a negative input terminal of subtractor 6. The positive input terminal of subtractor 6 is supplied with the current reproduced luminance signal from input terminal 1. Subtractor 6 subtracts the noise component from the current reproduced luminance signal. Thus, a reproduced luminance signal having the noise component reduced is provided via an output terminal 7.
With the noise reduction circuit of the non-recursive type of FIG. 1, not only the noise components but also the signal components not having line correlation are canceled from the reproduced luminance signal by subtractor 6. The resolution in the vertical direction of the reproduced picture is therefore degraded with pictures having blurred edges. Because this phenomenon is noticeable where noise reduction effect is increased, the SN ratio could not be improved sufficiently in the non-recursive type noise reduction circuit of FIG. 1.
A VTR has been proposed for simultaneously performing de-emphasis in the subtractor 13. The positive input terminal of subtractor 13 is supplied with the current reproduced luminance signal from input terminal 8, whereby subtractor 13 subtracts the component without line correlation from the current reproduced luminance signal. Thus, the reproduced luminance signal having the noise component reduced and the signal component without line correlation emphasized at the time of recording but now de-emphasized is applied to an output terminal 14, and to a 1H delay line 10. A series of the above mentioned operation is repeated thereafter.
The recursive type noise reduction circuit of FIG. 2 implements a closed-loop in which the reproduced luminance signal provided from output terminal 14 is fed-back to 1H delay line 10. The noise components are reduced according to the attenuation coefficient of attenuator 12, and the signal components without line correlation are de-emphasized to improve the SN ratio significantly.
The noise reduction rate in the recursive type noise reduction circuit of FIG. 2 is great because de-emphasis in the vertical direction and reduction of the noise components are simultaneously carried out with one circuit. This results in degradation of the resolution of the reproduced picture in the vertical direction as mentioned before. Also, the noise components having line correlation which were not reduced as described above is particularly emphasized, leading to a problem of raining-like trails in the vertical direction on the reproduced picture called "raining noise".